Natural draft cooling towers are predominantly used in the power generation industry for providing cooled circulating water to the condensers. The degree to which the circulating water is cooled by the cooling tower governs the performance of the condenser, the backpressure acting on the steam turbine, the turbine cycle heat rate and coal burnt for a given electricity output. Natural draft cooling towers can either be dry or wet, the more common type being wet, in which heat transfer from the circulating water being cooled, is largely through evaporation.
Natural draft cooling towers include a shell which is supported above a cold water collecting sump such that an annular air inlet opening is defined between the lower edge of the shell and the sump through which air can enter the shell. Air flow through the air inlet opening and upwardly through the shell is affected by a reduction in density of the column of air within the shell.
Tower performance or capability can be improved by replacing natural draft with forced draft utilizing fans. A number of different types of fan-assisted natural draft cooling towers have been designed and built. Fan mechanisms have been located in the air inlet opening with near horizontal fan shafts. In order to maintain or improve tower performance mechanical draft provided by the fans has to be equal to or greater that the natural draft it replaces, which requires large diameter fans. A disadvantage of this arrangement is that with fan blades positioned in a near vertical plane, they are directly subject to the adverse affect of wind and are at risk of destruction by high winds. U.K. patent 1 455 544 to Hamon—Sobelco (1975) attempts to overcome this problem by proposing fans located within the cooling tower shell. For an existing natural draft cooling tower, this proposal would require extensive modifications to the cooling tower and would be impractical and involve high cost.
U.K. patent 1 467 563 to Hamon—Sobelco (1975) and U.S. Pat. No. 3,903,212 to Lefevre propose fans located outside the cooling tower with vertical fan shafts, that is horizontally mounted fan blades that overcomes the risk of wind damage and makes the fans readily accessible. In both these patents it is envisaged that with the fans in operation, towers cannot operate in natural draft mode. U.S. Pat. No. 3,903,212 does allow operation in natural draft mode, but only when the fans are inoperative and air inlet louvers, which have to be closed for fan operation, are opened. For application to existing cooling towers both these proposals would also require extensive modification to the tower to provide the required external fan deck and for most applications this too would be impractical and involve high cost.
U.S. Pat. No. 4,164,256 to Kelp (1979) reverts to large diameter fans with near horizontal fan shafts located in the tower air inlet with closable openings located between the annular inlet fan openings. This arrangement does not overcome the problem of wind damage to the fan blades. It does however allow concurrent mechanical draft provided by the fans and natural draft air flowing through the closable openings. For application to an existing cooling tower, this proposal would also require extensive modification to the tower air inlet and furthermore suffers from the disadvantage of wind damage risk to the fan blades.
South African patent application 2003/1583 to KPE LLC (2003) provides a cooling tower which has a primary operating mode in which air is drawn into the tower by natural draft and a secondary operational mode in which air flow into the tower is supplemented by mechanical draft. In the secondary operational mode each air inlet path has a natural draft inlet through which air can be drawn by natural draft and spaced therefrom a supplementary inlet with which at least one fan is associated and through which air can be drawn into the air inlet path by operation of the fan. A disadvantage of this proposal is that it has to rely on the supplemental air provided by the fans to feed air under pressure into the cooling tower. To ensure that this occurs, the fans have to be configured when operating to discharge air into the air inlet opening at velocities of between 15 and 20 m/s.
This is a hit and miss approach. In the first instance, fan power is directly proportional to the cube of air velocity which results in high fan power requirements. Secondly it is assumed that provided sufficient air pressure is created by the fans, the natural draft will be supplemented without any regard to where the air into the tower is actually going. Simply sticking fans on to a natural draught cooling tower will not necessarily have the desired effect of improving cooling performance. Forcing cold air into a natural draft cooling tower without due consideration of hot water distribution within the tower and what the air is doing inside the tower could result in cold air by-pass through the heat transfer medium within the tower. If cold air by-pass occurs this has the effect of reducing natural draft and will reduce tower performance.
It is often the case that circumstances under which the power station generates and sells its power, changes, invariably requiring a higher output and better efficiency. These in turn lead to a requirement for better tower performance or capability. With wet cooling towers, improving tower performance results in increased liquid loss through evaporation.
While the amount of water lost through evaporation is a comparatively small percentage of the circulating water rate, typically about 1.5% to 2.0%, the quantity of circulating water is large, resulting in loss due to evaporation amounting to many millions of gallons per annum. In addition to evaporation losses from the cooling process, blow-out from the tower air inlet further increases the water lost from the system.
Blow-out losses occur on natural draft cooling towers of the counterflow type. Blow-out occurs, during windy conditions, at the base of the shell at the top of the air opening where water leaving the fill in the form of fine rain droplets, close to the perimeter of the shell, can easily be blown out of the tower. Furthermore, strong external wind can produce negative pressures on the leeward side of the tower, resulting in not only fine rain droplets being sucked out of the tower but also a loss of natural draft which adversely affects tower performance. Although not as significant as evaporation losses, blow-out loss can amount to as much as 0.2% of the cooling water circulation rate. To minimize this effect, wind shields have in the past been located around the base of the tower. However as wind shields should not restrict airflow into the tower, their effectiveness at eliminating blow-out has not been entirely successful.
In countries such as Australia and South Africa, water is a scarce commodity and is often the inhibiting factor in further power generation, unless expensive dry cooling is adopted. With increasing demands on existing power stations to improve efficiency and to generate more power, availability of water for cooling becomes a limiting factor.
In this specification, the terms “comprises”, “comprising” or similar terms are intended to mean a non-exclusive inclusion, such that a method, system or apparatus that comprises a list of elements does not include those elements solely, but may well include other elements not listed.